I've been making an OBD-II data logger with Arduino, which logs data like vehicle speed, engine RPM etc., to a SD card. With my OBD-II adapter for Arduino, this is not difficult implement. I used a small OLED display module to display status and some realtime data of the vehicle.

Currently it looks like this:

Logged data is rendered with HighCharts like following. Actually I am going to make the rendering a web service so everyone can view the data easily.

I have published my sketch and some more details on here on my website. The source code of this OBD-II data logger uses following Arduino libraries: